[Spec Update] Support inline metric definitions

[callum-mcdata]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward change to existing dbt-semantic-interfaces functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Say you want to define a derived metric that is a ratio of two measures. In the old world you'd use a ratio metric type but that will no longer exist. So you have two options:

• Define each measure as its own simple metric with create_metric: true
• ADDITION: Define the simple metrics inline with the derived metric definition.

metric:
  name: derived_metric
  type: derived
  type_params:
    expr: "metric_a / metric_b"
    metrics:
      - name: metric_a
        type: simple
        type_params:
         - measure: column_a

      - name: metric_b
        type: simple
        type_params:
         - measure: column_b

In the above example, metric_a and metric_b would NOT show up in list-metrics as they are only defined inline to derived_metric. Similar to how dbt uses ephemeral models today.

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response